As a matter of definition, an "optical image bar" comprises an array of optical picture element ("pixel") generators for converting a spatial pattern, which usually is represented by the information content of electrical input signals, into a corresponding optical intensity profile. Although there are a variety of applications for such devices in a number of different fields, a significant portion of the effort and expense that have been devoted to their development has been directed toward their application to electrophotographic printing, where they may prove to be a relatively low cost and reliable alternative to the flying spot raster scanners which have dominated that field since its inception. Another potentially important application for these image bars is in displays, although that possibility has received relatively little attention to date.
Several different types of image bars have been proposed, including electrically addressable LED arrays (see "Linear LED Array Has 300 Pixel/In. Resolution," Electronics Week, Jan. 21, 1985, p. 21), electro-mechanical spatial light modulators (see a commonly assigned U.S. Pat. No. 4,492,435 of M. E. Banton et al., which issued Jan. 8, 1985 on a "Multiple Array Full Width Electro Mechanical Modulator"), and electrooptic spatial light modulators (see another commonly assigned U.S. Pat. No. 4,281,904 of R. A. Sprague et al., which issued Aug. 4, 1981 on a "TIR Electro-Optic Modulator with Individually Addressable Electrodes"). Also see, "Light Gates Give Data Recorder Improved Hardcopy Resolution," Electronic Design, Jul. 19, 1979, pp. 31-32; "Polarizing Filters Plot Analog Waveforms," Machine Design, Vol. 51, No. 17, Jul. 26, 1979, p. 62; and "Data Recorder Eliminates Problem of Linearity," Design News, Feb. 4, 1980, pp. 56-57. Even though these image bars are based on diverse technologies, they share the common characteristic of having finite spatial addressing capacities (i.e. they are "discrete image bars") because there are only certain, predetermined coordinates ("addresses") in image space upon which they can center pixels. In other words, the image plane "footprint" of such an image bar envelopes a continuum of space, but the centers of the pixels are confined to certain discrete locations therein due to the limited addressing capacity of the image bar. For example, the addresses upon which pixels can be centered by a linear image bar of the foregoing type are laterally restricted. Consequently, there are spatial quantization errors which detract from the precision with which these image bars can locate pixels in an image plane, thereby tending to introduce unwanted spatial distortion into the image. Furthermore, if the image plane diameter of the individual pixels is less than their center-to-center displacement, the restricted addressing capacity of these image bars also causes interpixel intensity nulls.
Some of the more interesting image bar proposals are based on the use of TIR (total internal reflection) electrooptic spatial light modulators. In keeping with the teachings of a commonly assigned U.S. Pat. No. 4,396,252 of W. D. Turner, which issued Aug. 2, 1983 on "Proximity Coupled Electro-Optic Devices," such a modulator typically comprises a set of laterally separated, individually addressable electrodes which are maintained closely adjacent a reflective surface of an optically transparent electrooptic element, such as a lithium niobate crystal. In operation, substantially the full width of the electrooptic element is illuminated by a transversely collimated light beam. This light beam is applied to the electrooptic element at a near grazing angle of incidence with respect to its reflective surface and is brought to a wedge shaped focus on that surface, so that it is totally internally reflected therefrom. Moreover, voltages representing laterally adjacent pixels (i.e., a linear pixel pattern) are applied to the individually addressable electrodes, whereby localized fringe electric fields are coupled into the electrooptic element. These fields produce localized variations in the refractive index of the electrooptic element, so the wavefront of the light beam is spatially phase modulated in accordance with the pixel pattern as it passes through the electrooptic element. The process is repeated for a sequence of pixel patterns, with the result that the wavefront of the light beam is spatially modulated as a function of time in accordance with successive ones of those patterns. For image bar applications of such a modulator, Schlieren optics are employed to convert the phase modulated wavefront of the light beam into a corresponding series of optical intensity profiles. If a printing function is being performed, these intensity profiles are, in turn, used to expose a photosensitive recording medium, such as a xerographic photoreceptor, in accordance with the image defined by the successive pixel patterns.
There have been several significant developments which have reduced the cost and increased the reliability of TIR electrooptic image bars. Among these are a so-called "differential encoding" technique that is described in a commonly assigned U.S. Pat. No. 4,450,459 of W. D. Turner et al., which issued May 22, 1984 on "Differential Encoding for Fringe Field Responsive Electro-Optic Line Printers" and an electrical interconnect strategy that is described in a commonly assigned U.S. Pat. No. 4,367,925 of R. A. Spague et al., which issued Jan. 11, 1983 on "Integrated Electronics for Proximity Coupled Electro-Optic Devices." Briefly, it has been shown that the number of electrodes that a TIR electrooptic image bar requires to achieve a given resolution can be reduced by a factor of almost two if the input data samples (i.e., the electrical representations of the pixels to be printed) are differentially encoded, such that the magnitude of each of them, except for those that represent the initial pixels for the successive lines of the image, is referenced to the magnitude of the immediately preceeding sample. Additionally, it has been demonstrated that more or less conventional VLSI circuit technology may be employed to integrate the electrodes with their addressing and drive electronics, thereby promoting the orderly and reliable distribution of data samples to the large number of electrodes that ordinarily are required for reasonably high resolution printing.
Typically, the effective diameter of the pixels produced by an electrooptic image bar, as measured between their half power points at unity magnification, is approximately one half the center-to-center spacing of its electrodes. Accordingly, such image bars not only tend to cause image distortion because of spatial quantization errors, but also characteristically produce interpixel intensity nulls.
A copending and commonly assigned U.S. patent application of D. L. Hecht, which was filed 13 May 1985 under Ser. No. 733,354 now abandoned on "Discrete Image Bars Having Enhanced Spatial Addressing Capacity" (D/83038) teaches that the spatial addressing capacity of a discrete image bar may be increased by translating the position of its optical footprint laterally relative to its output image plane as a function of time, thereby enabling the image bar to incoherently superimpose on the image plane a plurality of independent pixel patterns which are laterally offset from one another by a distance that is less than the center-to-center spacing of the pixels of any one of those patterns. Also see, a commonly assigned U.S. Pat. No. 4,509,058 of K. H. Fischbeck, which issued Apr. 2, 1985 on "Ink Jet Printing Using Horizontal Interlacing" for a related concept as applied to ink jet printer arrays. These prior proposals have suggested the use of mechanical motion to enhance the spatial addressing capacity of discrete printing arrays, so it will be evident that the fundamental advantage of this invention is that it provides passive optics for performing that function, thereby avoiding the classical mechanical design problems of reliability, precision and repeatability. Another commonly assigned U.S. Pat. No. 4,483,596 of S. W. Marshall, which issued Nov. 20, 1984 on "Interference Suppression Apparatus and Method for a Linear Modulator," discloses a passive optical system for avoiding the destructive interference that can cause interpixel intensity nulls when using electrooptic image bars, but that proposal does not enhance the spatial addressing capacity of the image bar.